A typical nuclear steam generator comprises a vertically oriented shell, a plurality of U-shaped tubes disposed in the shell so as to form a tube bundle, a tube sheet for supporting the tubes at the ends opposite their U-like curvature, a dividing plate which is arranged with the tube sheet to form a primary fluid inlet header at one end of the tube bundle and a primary fluid outlet header at the other end of the tube bundle, a primary fluid inlet nozzle in fluid communication with the primary fluid inlet header and a primary fluid outlet nozzle in fluid communication with the primary fluid outlet header. The steam generator also comprises a wrapper sheet disposed between the tube bundle and the shell to form an annular chamber with the internal wall of the shell, and a feedwater ring disposed above the U-line curvature end of the tube bundle. The primary fluid having been heated by circulation through the reactor core enters the steam generator through the primary fluid inlet nozzle. From the primary fluid inlet nozzle, the primary fluid flows through the primary fluid inlet header, through the tubes of the bundle, out the primary fluid outlet header, through the primary fluid outlet nozzle to the remainder of the reactor coolant system. At the same time, feedwater is introduced to the steam generator through the feedwater ring. The feedwater is directed down the annular chamber adjacent to the shell until the tube sheet near the bottom of the annular chamber causes the feedwater to reverse direction passing in heat transfer relationship with the outside of the U-shaped tubes of the bundle and up through the inside of the wrapper. While the feedwater is circulating in heat transfer relationship with the tubes of the bundle, heat is transferred from the primary fluid in the tubes to the feedwater over the outside of the tubes, causing some predetermined portion of the feedwater to be converted to steam. The steam then rises and is circulated through typical electrical generating equipment producing electricity in a manner well-known in the art.
Since the primary fluid contains radioactive particles and is isolated from the feedwater only by the walls of the U-shaped tubes which may be constructed from Inconel, the U-tube walls form part of the primary boundary for isolating these radioactive particles. It is, therefore, important that the U-tubes be maintained defect-free so that no ruptures will occur in the U-tubes. However, experience has shown that under certain conditions the U-tubes may develop leaks therein which allow radioactive particles to contaminate the feedwater, a highly undesirable accident.
There is thought to be several causes of tube leaks in steam generators. One cause of these leaks is considered to be related to the chemical environment on the feedwater side of the tubes. Analysis of tube samples taken from operating steam generators which have experienced leaks has shown that the leaks were caused by cracks in the tubes resulting from intergranular corrosion. High caustic levels found in the vicinity of the cracks in the tube specimens taken from operating steam generators, and the similarity of these cracks to failures produced by caustic under controlled laboratory conditions, have identified high caustic levels as a cause of the intergranular corrosion and thus the cause of the tube cracking.
Another cause of tube leaks is inferred to be from tube thinning. Eddy current tests of the tubes have indicated that the thinning occurs on the tubes near the tube sheet at levels corresponding to the levels of sludge that has accumulated on the tube sheet. The sludge is mainly iron oxides and copper compounds along with traces of other metals that have settled out of the feedwater onto the tube sheet. The level of sludge accumulation may be inferred from eddy current testing with a low frequency signal that is sensitive to the magnetite in the sludge. The correlation between sludge levels and tube wall thinning locations strongly implies that the sludge deposits provide a site for concentration of a phosphate solution or other corrosive agents at the tube wall that result in tube thinning.
Further, more recent experience has discovered that tube failures are brought about by stress corrosion, cracking and pitting. As a matter of fact, pitting was the only problem identified by the Assignor of this Application at Millstone II.
Regardless of the specific reason for tube failure in the area above the tube sheet. the present invention is concerned with the removal of the so-called sludge which has certainly been pinned down as creating an environment for attack on the tubes. It is this material which must be liquified by having fluid directly injected into it, preparatory to being flushed from the tube sheet. One known method for removal of this sludge is referred to as sludge lancing. Sludge lancing includes using high pressure water to break up and slurry the sludge in conjunction with suction and filtration equipment that removes the water/sludge mixture for disposal or recirculation. An excellent discussion of the background of this system is disclosed in U.S. Pat. No. 4,079,701, Robert A. Hickman, et al., issued Mar. 21, 1978. All of the problems of this system center ground the removal of sludge by the mechanical arrangement of lance manipulation to drive the sludge into a suction header. If a transport apparatus can be moved into the confined space at the periphery of the tube bundle, a sludge lance can be mounted on the transporter and manipulated as required to fluidize the sludge in preparation for its removal.
The present problem is in moving the transport apparatus into the limited area between the periphery of the tube bundle and the internal wall of the shell. Once the transporter has been moved into position at the bundle periphery, another problem is movement of the transporter along the periphery and periodic immobilization of the transporter to facilitate manipulation of the sludge lance and/or inspection apparatus mounted on the transporter.